If your torch is getting hot, see if you can disconnect the hoses off the torch and see if it's clogged. When I set up a job I almost always spend the first day screwing around with getting good water flow because of alge buildup in the tiny little passage ways in my weld heads.
If you go shurflo or flowjet, you will find some 12v options intended to be run off an automotive battery. Not sure if you're doing alot of field work or not, but it's an option.
I went back and double checked some specs. This is probably very over kill for what you're doing, but it's nice to see what's out there. Procon Pumps is the manufacturer who makes the pump in the machines I run the most (hardest and longest, continuous for hours sort of welds), they have a section on their site with pumps that are used for welding applications. The series 1 pump is what is in my machines, it's set for 60 psi and according to the specs it's low end low rate is 15 gpm. If you scroll down, they've got a section called "chillers and heat exchangers". I don't see anything that looks like a chiller or a heat exchanger, but I see a "Mag 2" option which has the pump and the motor shown, and a picture of a guy welding (whatever that means). It looks to have comparable pressure and flow rates to the Series 1 pump.
While you're setting up a system, I'd definitely encourage getting some in-line filters to protect whatever pump you get and keep junk out of the torch body. The Procon pumps have what looks to be a 200 mesh (or there abouts) filters on them. Placing some courser filters in line (like maybe 80 mesh) certainly will help keep the miles down on the finer mesh filter (that you can't see and requires a wrench to get to). The shurflo and flojet pumps are just wide open, they don't have a built in filter. Something like this is all you need:
http://www.blowsion.com/products/54-water-filter-inline-38.aspx And last but not least, when I was shopping for pumps for my personal stuff, eBay was a savior. Really good prices on new or lightly used stuff. Not sure if it's the same way now, but worth maybe checking out.